The present invention relates to a novel negatively chargeable dry toner for developing electrostatic latent images for use in electrophotography, electrostatic recording, electrostatic printing, etc.
Electrostatic latent images can be developed into visible images with a toner deposited thereon by electrostatic attraction. Powder developers as well as liquid developers are widely used for developing electrostatic latent images.
Powder developers can be divided generally into two-component developers and single-component developers. The two-component developer comprises a finely divided toner having a mean particle size of 15 .mu.m and prepared by dispersing a coloring agent, charge control agent, fluidizing agent and the like in a natural or synthetic resin, and a carrier of finely divided iron, ferrite or the like admixed with the toner and 100 to 200 .mu.m in particle size. The latter single-component developer comprises only a finely divided toner having a mean particle size of 15 .mu.m and prepared by dispersing a coloring agent, charge control agent, fluidizing agent, magnetic material and the like in a natural or synthetic resin.
Electrostatic latent images are developed with the two-component developer by triboelectrically charging the toner with the carrier and depositing the toner on the latent image. Toners heretofore known and serving as single-component developers include those which are triboelectrically chargeable by a brushlike or platelike friction member used in place of the carrier and having the same function as the carrier. Further provided in recent years are toners which are triboelectrically chargeable by a finely divided magnetic material which is maintained in a dispersed state. These developing toners are charged positively or negatively in accordance with the polarity of the electrostatic latent image to be developed.
To enable the toner to retain the charge, it is also proposed to utilize the triboelectric chargeability of the resin used as the main component of the toner, but the toner so adapted is low in chargeability and has a great solid surface resistance value. Consequently the toner image obtained is prone to fogging and obscure. To impart the desired chargeability to toners, it is practice to add to the toner a charge imparting dye or pigment, and a charge control agent. Presently used in the art are oil-soluble nigrosine dyes for imparting a positive charge to the toner as disclosed in Examined Japanese Patent Publication No. SHO 41-2427, etc., and metal-containing complex salt dyes for giving a negative charge as disclosed in Examined Japanese Patent Publications No. SHO 41-20153, No. SHO 43-17955 and No. SHO 45-26478, etc.
However, such dyes or pigments serving as charge control agents are complex in structure and low in stability. For example, they are liable to decompose or degrade, failing to exhibit charge control ability when subjected to mechanical friction and impact, to changes in temperature or humidity or to electric impact, or when exposed to light. Furthermore, they have a substantial defect in that being colored substances, they fail to fulfill the requirement that the charge control agent should be colorless or substantially colorless when to be used for a toner of particular color.
Recently, various charge control agents have been disclosed which meet this requirement. Among these, the compounds disclosed in Examined Japanese Patent Publications No. SHO 55-42452, No. SHO 58-41508, No. SHO 59-7348 and No. SHO 59-26944 contain chromium, cobalt or like heavy metal, while those disclosed in Unexamined Japanese Patent Publications No. SHO 61-69073 and No. SHO 61-73963 contain zinc. These compounds therefore leave the problem to be solved of providing a charge control agent free from heavy metal.